Substrate-specific binding of hook-associated proteins by FlgN and FliT, putative chaperones for flagellum assembly

A novel C-terminal region within the multicargo type III secretion chaperone CesT contributes to effector secretion

The enteropathogenic Escherichia coli (EPEC) multicargo chaperone CesT interacts with at least 10 effector proteins and is central to pathogenesis. CesT has been implicated in coordinating effector hierarchy, although the mechanisms behind this regulation are poorly understood. To address this question, we set out to functionally characterize CesT with respect to roles in (i) effector binding, (ii) effector recruitment to the type III secretion system (T3SS), and (iii) effector translocation into host cells. A CesT variant expression library was screened in EPEC using a newly developed semi-high-throughput secretion assay. Among many deficient CesT variants, a predominant number were localized to a novel CesT C-terminal region. These CesT C-terminal variants exhibited normal effector binding yet reduced effector secretion levels. Structural correlation and thermal spectroscopy analyses of purified CesT variants implicated multiple surface-exposed residues, a terminal helix region, and a flexible C-terminal triple-serine stretch in effector secretion. Site-directed mutagenesis of the flexible CesT C-terminal triple-serine sequence produced differential effector secretion, implicating this region in secretion events. Infection assays further indicated that the C-terminal region of CesT was important for NleA translocation into host cells but was dispensable for Tir translocation. The findings implicate the CesT C terminus in effector secretion and contribute to a model for multiple-cargo chaperone function and effector translocation into host cells during infection.

Protein export according to schedule: architecture, assembly, and regulation of type III secretion systems from plant- and animal-pathogenic bacteria

Flagellar and translocation-associated type III secretion (T3S) systems are present in most gram-negative plant- and animal-pathogenic bacteria and are often essential for bacterial motility or pathogenicity. The architectures of the complex membrane-spanning secretion apparatuses of both systems are similar, but they are associated with different extracellular appendages, including the flagellar hook and filament or the needle/pilus structures of translocation-associated T3S systems. The needle/pilus is connected to a bacterial translocon that is inserted into the host plasma membrane and mediates the transkingdom transport of bacterial effector proteins into eukaryotic cells. During the last 3 to 5 years, significant progress has been made in the characterization of membrane-associated core components and extracellular structures of T3S systems. Furthermore, transcriptional and posttranscriptional regulators that control T3S gene expression and substrate specificity have been described. Given the architecture of the T3S system, it is assumed that extracellular components of the secretion apparatus are secreted prior to effector proteins, suggesting that there is a hierarchy in T3S. The aim of this review is to summarize our current knowledge of T3S system components and associated control proteins from both plant- and animal-pathogenic bacteria.

Complete Genome of Ignavibacterium album, a Metabolically Versatile, Flagellated, Facultative Anaerobe from the Phylum Chlorobi

Prior to the recent discovery of Ignavibacterium album (I. album), anaerobic photoautotrophic green sulfur bacteria (GSB) were the only members of the bacterial phylum Chlorobi that had been grown axenically. In contrast to GSB, sequence analysis of the 3.7-Mbp genome of I. album shows that this recently described member of the phylum Chlorobi is a chemoheterotroph with a versatile metabolism. I. album lacks genes for photosynthesis and sulfur oxidation but has a full set of genes for flagella and chemotaxis. The occurrence of genes for multiple electron transfer complexes suggests that I. album is capable of organoheterotrophy under both oxic and anoxic conditions. The occurrence of genes encoding enzymes for CO(2) fixation as well as other enzymes of the reductive TCA cycle suggests that mixotrophy may be possible under certain growth conditions. However, known biosynthetic pathways for several amino acids are incomplete; this suggests that I. album is dependent upon on exogenous sources of these metabolites or employs novel biosynthetic pathways. Comparisons of I. album and other members of the phylum Chlorobi suggest that the physiology of the ancestors of this phylum might have been quite different from that of modern GSB.

Expanded roles for multicargo and class 1B effector chaperones in type III secretion

Bacterial type III secretion systems (T3SS) are complex protein assemblies that mediate the secretion of protein substrates outside the cell. Type III secretion chaperones (T3SC) are always found associated with T3SS, and they serve in multiple roles to ensure that protein substrates are efficiently targeted for secretion. Bacterial pathogens with T3SS express T3SC proteins that bind effectors, a process important for effector protein delivery into eukaryotic cells during infection. In this minireview, we focus on multicargo and class 1B T3SC that associate with effectors within significant pathogens of animals and plants. As a primary role, multicargo and class 1B T3SC form homodimers and specifically bind different effectors within the cytoplasm, maintaining the effectors in a secretion-competent state. This role makes T3SC initial and central contributors to effector-mediated pathogenesis. Recent findings have greatly expanded our understanding of cellular events linked to multicargo T3SC function. New binding interactions with T3SS components have been reported in different systems, thereby implicating multicargo T3SC in critical roles beyond effector binding. Three notable interactions with the YscN, YscV, and YscQ family members are well represented in the literature. Similar T3SC interactions are reported in the putative related flagellar T3SS, suggesting that secretion mechanisms may be more similar than previously thought. The evidence implicates multicargo and class 1B T3SC in effector binding and stabilization, in addition to T3SS recruitment and docking events.

The flagellar regulator fliT represses Salmonella pathogenicity island 1 through flhDC and fliZ

Salmonella pathogenicity island 1 (SPI1), comprising a type III section system that translocates effector proteins into host cells, is essential for the enteric pathogen Salmonella to penetrate the intestinal epithelium and subsequently to cause disease. Using random transposon mutagenesis, we found that a Tn10 disruption in the flagellar fliDST operon induced SPI1 expression when the strain was grown under conditions designed to repress SPI1, by mimicking the environment of the large intestine through the use of the intestinal fatty acid butyrate. Our genetic studies showed that only fliT within this operon was required for this effect, and that exogenous over-expression of fliT alone significantly reduced the expression of SPI1 genes, including the invasion regulator hilA and the sipBCDA operon, encoding type III section system effector proteins, and Salmonella invasion of cultured epithelial cells. fliT has been known to inhibit the flagellar machinery through repression of the flagellar master regulator flhDC. We found that the repressive effect of fliT on invasion genes was completely abolished in the absence of flhDC or fliZ, the latter previously shown to induce SPI1, indicating that this regulatory pathway is required for invasion control by fliT. Although this flhDC-fliZ pathway was necessary for fliT to negatively control invasion genes, fliZ was not essential for the repressive effect of fliT on motility, placing fliT high in the regulatory cascade for both invasion and motility.

YdiV: a dual function protein that targets FlhDC for ClpXP-dependent degradation by promoting release of DNA-bound FlhDC complex

YdiV is an EAL-like protein that acts as a post-transcriptional, negative regulator of the flagellar master transcriptional activator complex, FlhD(4)C(2), in Salmonella enterica to couple flagellar gene expression to nutrient availability. Mutants defective in ClpXP protease no longer exhibit YdiV-dependent inhibition of FlhD(4)C(2)-dependent transcription under moderate YdiV expression conditions. ClpXP protease degrades FlhD(4)C(2), and this degradation is accelerated in the presence of YdiV. YdiV complexed with both free and DNA-bound FlhD(4)C(2); and stripped FlhD(4)C(2) from DNA. A L22H substitution in FlhD was isolated as insensitive to YdiV inhibition. The FlhD L22H substitution prevented the interaction of YdiV with free FlhD(4)C(2) and the ability of YdiV to release FlhD(4)C(2) bound to DNA. These results demonstrate that YdiV prevents FlhD(4)C(2)-dependent flagellar gene transcription and acts as a putative adaptor to target FlhD(4)C(2) for ClpXP-dependent proteolysis. Our results suggest that YdiV is an EAL-like protein that has evolved from a dicyclic-GMP phosphodiesterase into a dual-function regulatory protein that connects flagellar gene expression to nutrient starvation.

Crystallization and preliminary X-ray analysis of FlgA, a periplasmic protein essential for flagellar P-ring assembly

Salmonella FlgA, a periplasmic protein essential for flagellar P-ring assembly, has been crystallized in two forms. The native protein crystallized in space group C222, with unit-cell parameters a = 107.5, b = 131.8, c = 49.4 Å, and diffracted to about 2.0 Å resolution (crystal form I). In this crystal, the asymmetric unit is likely to contain one molecule, with a solvent content of 66.8%. Selenomethionine-labelled FlgA protein crystallized in space group C222(1), with unit-cell parameters a = 53.2, b = 162.5, c = 103.5 Å, and diffracted to 2.7 Å resolution (crystal form II). In crystal form II, the asymmetric unit contained two molecules with a solvent content of 48.0%. The multiple-wavelength and single-wavelength anomalous dispersion methods allowed the visualization of the electron-density distributions of the form I and II crystals, respectively. The two maps suggested that FlgA is in two different conformations in the two crystals.

A single-domain FlgJ contributes to flagellar hook and filament formation in the Lyme disease spirochete Borrelia burgdorferi

FlgJ plays a very important role in flagellar assembly. In the enteric bacteria, flgJ null mutants fail to produce the flagellar rods, hooks, and filaments but still assemble the integral membrane-supramembrane (MS) rings. These mutants are nonmotile. The FlgJ proteins consist of two functional domains. The N-terminal rod-capping domain acts as a scaffold for rod assembly, and the C-terminal domain acts as a peptidoglycan (PG) hydrolase (PGase), which allows the elongating flagellar rod to penetrate through the PG layer. However, the FlgJ homologs in several bacterial phyla (including spirochetes) often lack the PGase domain. The function of these single-domain FlgJ proteins remains elusive. Herein, a single-domain FlgJ homolog (FlgJ(Bb)) was studied in the Lyme disease spirochete Borrelia burgdorferi. Cryo-electron tomography analysis revealed that the flgJ(Bb) mutant still assembled intact flagellar basal bodies but had fewer and disoriented flagellar hooks and filaments. Consistently, Western blots showed that the levels of flagellar hook (FlgE) and filament (FlaB) proteins were substantially decreased in the flgJ(Bb) mutant. Further studies disclosed that the decreases of FlgE and FlaB in the mutant occurred at the posttranscriptional level. Microscopic observation and swarm plate assay showed that the motility of the flgJ(Bb) mutant was partially deficient. The altered phenotypes were completely restored when the mutant was complemented. Collectively, these results indicate that FlgJ(Bb) is involved in the assembly of the flagellar hook and filament but not the flagellar rod in B. burgdorferi. The observed phenotype is different from that of flgJ mutants in the enteric bacteria.

Proteus mirabilis interkingdom swarming signals attract blow flies

Flies transport specific bacteria with their larvae that provide a wider range of nutrients for those bacteria. Our hypothesis was that this symbiotic interaction may depend on interkingdom signaling. We obtained Proteus mirabilis from the salivary glands of the blow fly Lucilia sericata; this strain swarmed significantly and produced a strong odor that attracts blow flies. To identify the putative interkingdom signals for the bacterium and flies, we reasoned that as swarming is used by this bacterium to cover the food resource and requires bacterial signaling, the same bacterial signals used for swarming may be used to communicate with blow flies. Using transposon mutagenesis, we identified six novel genes for swarming (ureR, fis, hybG, zapB, fadE and PROSTU_03490), then, confirming our hypothesis, we discovered that fly attractants, lactic acid, phenol, NaOH, KOH and ammonia, restore swarming for cells with the swarming mutations. Hence, compounds produced by the bacterium that attract flies also are utilized for swarming. In addition, bacteria with the swarming mutation rfaL attracted fewer blow flies and reduced the number of eggs laid by the flies. Therefore, we have identified several interkingdom signals between P. mirabilis and blow flies.

Swarming motility and the control of master regulators of flagellar biosynthesis

Swarming motility is the movement of bacteria over a solid surface powered by rotating flagella. The expression of flagellar biosynthesis genes is governed by species-specific master regulator transcription factors. Mutations that reduce or enhance master regulator activity have a commensurate effect on swarming motility. Here we review what is known about the proteins that modulate swarming motility and appear to act upstream of the master flagellar regulators in diverse swarming bacteria. We hypothesize that environmental control of the master regulators is important to the swarming phenotype perhaps at the level of controlling flagellar number.

Escherichia coli genome-wide promoter analysis: identification of additional AtoC binding target elements

Background

Studies on bacterial signal transduction systems have revealed complex networks of functional interactions, where the response regulators play a pivotal role. The AtoSC system of E. coli activates the expression of atoDAEB operon genes, and the subsequent catabolism of short-chain fatty acids, upon acetoacetate induction. Transcriptome and phenotypic analyses suggested that atoSC is also involved in several other cellular activities, although we have recently reported a palindromic repeat within the atoDAEB promoter as the single, cis-regulatory binding site of the AtoC response regulator. In this work, we used a computational approach to explore the presence of yet unidentified AtoC binding sites within other parts of the E. coli genome.

Results

Through the implementation of a computational de novo motif detection workflow, a set of candidate motifs was generated, representing putative AtoC binding targets within the E. coli genome. In order to assess the biological relevance of the motifs and to select for experimental validation of those sequences related robustly with distinct cellular functions, we implemented a novel approach that applies Gene Ontology Term Analysis to the motif hits and selected those that were qualified through this procedure. The computational results were validated using Chromatin Immunoprecipitation assays to assess the in vivo binding of AtoC to the predicted sites. This process verified twenty-two additional AtoC binding sites, located not only within intergenic regions, but also within gene-encoding sequences.

Conclusions

This study, by tracing a number of putative AtoC binding sites, has indicated an AtoC-related cross-regulatory function. This highlights the significance of computational genome-wide approaches in elucidating complex patterns of bacterial cell regulation.

The interaction dynamics of a negative feedback loop regulates flagellar number in Salmonella enterica serovar Typhimurium

Each Salmonella enterica serovar Typhimurium cell produces a discrete number of complete flagella. Flagellar assembly responds to changes in growth rates through FlhD(4) C(2) activity. FlhD(4) C(2) activity is negatively regulated by the type 3 secretion chaperone FliT. FliT is known to interact with the flagellar filament cap protein FliD as well as components of the flagellar type 3 secretion apparatus. FliD is proposed to act as an anti-regulator, in a manner similar to FlgM inhibition of σ(28) activity. We have found that efficient growth-dependent regulation of FlhD(4) C(2) requires FliT regulation. In turn, FliD regulation of FliT modulates the response. We also show that, unlike other flagellar-specific regulatory circuits, deletion of fliT or fliD did not lead to an all-or-nothing response in FlhD(4) C(2) activity. To investigate why, we characterized the biochemical interactions in the FliT : FliD :  FlhD(4) C(2) circuit. When FlhD(4) C(2) was not bound to DNA, FliT disrupted the FlhD(4) C(2) complex. Interestingly, when FlhD(4) C(2) was bound to DNA it was insensitive to FliT regulation. This suggests that the FliT circuit regulates FlhD(4) C(2) activity by preventing the formation of the FlhD(4) C(2) :DNA complex. Our data would suggest that this level of endogenous regulation of FlhD(4) C(2) activity allows the flagellar system to efficiently respond to external signals.

Multiple promoters contribute to swarming and the coordination of transcription with flagellar assembly in Salmonella

In Salmonella, there are three classes of promoters in the flagellar transcriptional hierarchy. This organization allows genes needed earlier in the construction of flagella to be transcribed before genes needed later. Four operons (fliAZY, flgMN, fliDST, and flgKL) are expressed from both class 2 and class 3 promoters. To investigate the purpose for expressing genes from multiple flagellar promoters, mutants were constructed for each operon that were defective in either class 2 transcription or class 3 transcription. The mutants were checked for defects in swimming through liquids, swarming over surfaces, and transcriptional regulation. The expression of the hook-associated proteins (FlgK, FlgL, and FliD) from class 3 promoters was found to be important for swarming motility. Both flgMN promoters were involved in coordinating class 3 transcription with the stage of assembly of the hook-basal body. Finally, the fliAZY class 3 promoter lowered class 3 transcription in stationary phase. These results indicate that the multiple flagellar promoters respond to specific environmental conditions and help coordinate transcription with flagellar assembly.

Molecular interaction of flagellar export chaperone FliS and cochaperone HP1076 in Helicobacter pylori

Flagellar export chaperone FliS prevents premature polymerization of flagellins and is critical for flagellar assembly and bacterial colonization. Previously, a yeast 2-hybrid study identified various FliS-associated proteins in Helicobacter pylori, but the implications of these interactions are not known. Here we demonstrate the biophysical interaction of FliS (HP0753) and the uncharacterized protein HP1076 from H. pylori. HP1076 possesses a cochaperone activity that promotes the folding and chaperone activity of FliS. We further determined the crystal structures of FliS, HP1076, and the binary complex at 2.7, 1.8, and 2.7 Å resolution, respectively. HP1076 adopts a helix-rich bundle structure and interestingly shares a similar fold with a flagellin homologue, hook-associated protein, and FliS. The FliS-HP1076 complex revealed an extensive electrostatic and hydrophobic binding interface, which is distinct from the flagellin binding pocket in FliS. The helical stacking interaction between HP1076 and FliS suggests that HP1076 stabilizes 2 α helices of FliS and therefore the overall structure of the bundle. Our findings provide new insights into flagellar export chaperones and may have implications for other secretion chaperones in the type III secretion system.

FlhA provides the adaptor for coordinated delivery of late flagella building blocks to the type III secretion system

Flagella are the bacterial organelles of motility and can play important roles in pathogenesis. Flagella biosynthesis requires the coordinated export of huge protein amounts from the cytosol to the nascent flagellar structure at the cell surface and employs a type III secretion system (T3SS). Here we show that the integral membrane protein FlhA from the gram-positive bacterium Bacillus subtilis acts as an adaptor for late export substrates at the T3SS. The major filament protein (flagellin) and the filament-cap protein (FliD) bind to the FlhA cytoplasmic domain (FlhA-C) only in complex with their cognate chaperones (FliS and FliT). To understand the molecular details of these interactions we determined the FlhA-C crystal structure at 2.3 A resolution. FlhA-C consists of an N-terminal linker region, three subdomains with a novel fold, and a disordered region essential for the adaptor function. We show that the export protein FliJ associates with the linker region and modulates the binding properties of FlhA-C. While the interaction of FliD/FliT is enhanced, flagellin/FliS is not affected. FliJ also keeps FliT associated with FlhA-C and excess of FliT inhibits binding of FliD/FliT, suggesting that empty FliT chaperones stay associated with FliJ after export of FliD. Taken together, these results allow to propose a model that explains how the T3SS may switch from the stoichiometric export of FliD to the high-throughput secretion of flagellin.

Structural insight into the regulatory mechanisms of interactions of the flagellar type III chaperone FliT with its binding partners

For self-assembly of the bacterial flagellum, most of the flagellar component proteins synthesized in the cytoplasm are exported by the flagellar type III export apparatus to the growing, distal end. Flagellar protein export is highly organized and well controlled in every step of the flagellar assembly process. Flagellar-specific chaperones not only facilitate the export of their cognate proteins, as well as prevent their premature aggregation in the cytoplasm, but also play a role in fine-tuning flagellar gene expression to be coupled with the flagellar assembly process. FliT is a flagellar-specific chaperone responsible for the export of the filament-capping protein FliD and for negative control of flagellar gene expression by binding to the FlhDC complex. Here we report the crystal structure of Salmonella FliT at 3.2-A resolution. The structural and biochemical analyses clearly reveal that the C-terminal segment of FliT regulates its interactions with the FlhDC complex, FliI ATPase, and FliJ (subunits of the export apparatus), and that its conformational change is responsible for the switch in its binding partners during flagellar protein export.

The HP0256 gene product is involved in motility and cell envelope architecture of Helicobacter pylori

BACKGROUND

Helicobacter pylori is the causative agent for gastritis, and peptic and duodenal ulcers. The bacterium displays 5-6 polar sheathed flagella that are essential for colonisation and persistence in the gastric mucosa. The biochemistry and genetics of flagellar biogenesis in H. pylori has not been fully elucidated. Bioinformatics analysis suggested that the gene HP0256, annotated as hypothetical, was a FliJ homologue. In Salmonella, FliJ is a chaperone escort protein for FlgN and FliT, two proteins that themselves display chaperone activity for components of the hook, the rod and the filament.

RESULTS

Ablation of the HP0256 gene in H. pylori significantly reduced motility. However, flagellin and hook protein synthesis was not affected in the HP0256 mutant. Transmission electron transmission microscopy revealed that the HP0256 mutant cells displayed a normal flagellum configuration, suggesting that HP0256 was not essential for assembly and polar localisation of the flagella in the cell. Interestingly, whole genome microarrays of an HP0256 mutant revealed transcriptional changes in a number of genes associated with the flagellar regulon and the cell envelope, such as outer membrane proteins and adhesins. Consistent with the array data, lack of the HP0256 gene significantly reduced adhesion and the inflammatory response in host cells.

CONCLUSIONS

We conclude that HP0256 is not a functional counterpart of FliJ in H. pylori. However, it is required for full motility and it is involved, possibly indirectly, in expression of outer membrane proteins and adhesins involved in pathogenesis and adhesion.

Purification, crystallization and preliminary X-ray analysis of FliT, a bacterial flagellar substrate-specific export chaperone

The assembly process of the bacterial flagellum is coupled to flagellar gene expression. FliT acts not only as a flagellar type III substrate-specific export chaperone for the filament-capping protein FliD but also as a negative regulator that suppresses flagellar gene expression through its specific interaction with the master regulator FlhD(4)C(2) complex. In this study, FliT of Salmonella enterica serovar Typhimurium was expressed, purified and crystallized. Crystals of SeMet FliT were obtained by the sitting-drop vapour-diffusion technique with potassium/sodium tartrate as the precipitant. The crystals grew in the trigonal space group P3(1)21 or P3(2)21 and diffracted to 3.2 A resolution. The anomalous difference Patterson map of the SeMet FliT crystal showed significant peaks in its Harker sections, indicating the usefulness of the derivative data for structure determination.

Type III secretion: a secretory pathway serving both motility and virulence (Review)

'Type III secretion' (T3S) refers to a secretion pathway that is common to the flagellae of eubacteria and the injectisomes of some gram-negative bacteria. Flagellae are rotary nanomachines allowing motility but they contain a built-in secretion apparatus that exports their own distal components to the distal end of the growing structure where they polymerize. In some cases they have been shown to export non-flagellar proteins. Injectisomes are transkingdom communication apparatuses allowing bacteria docked at the surface of a eukaryotic cell membrane to inject effector proteins across the two bacterial membranes and the eukaryotic cell membrane. Both nanomachines share a similar basal body embedded in the two bacterial membranes, topped either by a hook and a filament or by a stiff short needle. Both appear to be assembled in the same fashion. They recognize their substrate by a loose N-terminal peptide signal and the help of individual chaperones of a new type.

Mechanisms of type III protein export for bacterial flagellar assembly

Flagellar type III protein export is highly organized and well controlled in a timely manner by dynamic, specific and cooperative interactions among components of the export apparatus, allowing the huge and complex macromolecular assembly to be built efficiently. The bacterial flagellum, which is required for motility, consists of a rotary motor, a universal joint and a helical propeller. Most of the flagellar components are translocated to the distal, growing end of the flagellum for assembly through the central channel of the flagellum itself by the flagellar type III protein export apparatus, which is postulated to be located on the cytoplasmic side of the flagellar basal body. The export specificity switching machinery, which consists of at least two proteins that function as a molecular ruler and an export switch, respectively, monitors the state of hook-basal body assembly in the cell exterior and switches export specificity, thereby coupling sequential flagellar gene expression with the flagellar assembly process. The export ATPase complex composed of an ATPase and its regulator acts as a pilot to deliver its export substrate to the export gate and helps initial entry of the substrate N-terminal chain into a narrow pore of the export gate. The energy of ATP hydrolysis appears to be used to disassemble and release the ATPase complex from the protein about to be exported, and the rest of the successive unfolding/translocation process of the long polypeptide chain is driven solely by proton motive force (PMF), perhaps through biased one-dimensional Brownian diffusion. Interestingly, the subunits of the ATPase complex have significant sequence similarities to subunits of F(0)F(1)-ATP synthase, a rotary motor that drives the chemical reaction of ATP synthesis using PMF, and the entire crystal structure of the export ATPase is extremely similar to the alpha/beta subunits of F(0)F(1)-ATP synthase, suggesting that the flagellar export apparatus and F(0)F(1)-ATP synthase share the mechanism for their two distinct functions.

The rate of protein secretion dictates the temporal dynamics of flagellar gene expression

Flagellar gene expression is temporally regulated in response to the assembly state of the growing flagellum. The key mechanism for enforcing this temporal hierarchy in Salmonella enterica serovar Typhimurium is the sigma(28)-FlgM checkpoint, which couples the expression of the late flagellar (P(class3)) genes to the completion of the hook-basal body. This checkpoint is triggered when FlgM is secreted from the cell. In addition to the sigma(28)-FlgM checkpoint, a number of other regulatory mechanisms respond to the secretion of late proteins. In this work, we examined how middle (P(class2)) and late (P(class3)) gene expression is affected by late protein secretion. Dynamic analysis of flagellar gene expression identified a novel mechanism where induction of P(class2) activity is delayed either when late protein secretion is abolished or when late protein secretion is increased. Using a number of different approaches, we were able to show that this mechanism did not involve any known flagellar regulator. Furthermore, the changes in P(class2) activity were not correlated with the associated changes in P(class3) activity, which was found to be proportional to late protein secretion rates. Our data indicate that both P(class2) and P(class3) promoters are continuously regulated in response to assembly and late protein secretion rates. These results suggest that flagellar regulation is more complex than previously thought.

FliZ Is a posttranslational activator of FlhD4C2-dependent flagellar gene expression

Flagellar assembly proceeds in a sequential manner, beginning at the base and concluding with the filament. A critical aspect of assembly is that gene expression is coupled to assembly. When cells transition from a nonflagellated to a flagellated state, gene expression is sequential, reflecting the manner in which the flagellum is made. A key mechanism for establishing this temporal hierarchy is the sigma(28)-FlgM checkpoint, which couples the expression of late flagellar (P(class3)) genes to the completion of the hook-basal body. In this work, we investigated the role of FliZ in coupling middle flagellar (P(class2)) gene expression to assembly in Salmonella enterica serovar Typhimurium. We demonstrate that FliZ is an FlhD(4)C(2)-dependent activator of P(class2)/middle gene expression. Our results suggest that FliZ regulates the concentration of FlhD(4)C(2) posttranslationally. We also demonstrate that FliZ functions independently of the flagellum-specific sigma factor sigma(28) and the filament-cap chaperone/FlhD(4)C(2) inhibitor FliT. Furthermore, we show that the previously described ability of sigma(28) to activate P(class2)/middle gene expression is, in fact, due to FliZ, as both are expressed from the same overlapping P(class2) and P(class3) promoters at the fliAZY locus. We conclude by discussing the role of FliZ regulation with respect to flagellar biosynthesis based on our characterization of gene expression and FliZ's role in swimming and swarming motility.

Flagellar motility in bacteria structure and function of flagellar motor

Bacterial flagella are filamentous organelles that drive cell locomotion. They thrust cells in liquids (swimming) or on surfaces (swarming) so that cells can move toward favorable environments. At the base of each flagellum, a reversible rotary motor, which is powered by the proton- or the sodium-motive force, is embedded in the cell envelope. The motor consists of two parts: the rotating part, or rotor, that is connected to the hook and the filament, and the nonrotating part, or stator, that conducts coupling ion and is responsible for energy conversion. Intensive genetic and biochemical studies of the flagellum have been conducted in Salmonella typhimurium and Escherichia coli, and more than 50 gene products are known to be involved in flagellar assembly and function. The energy-coupling mechanism, however, is still not known. In this chapter, we survey our current knowledge of the flagellar system, based mostly on studies from Salmonella, E. coli, and marine species Vibrio alginolyticus, supplemented with distinct aspects of other bacterial species revealed by recent studies.

Sorting of early and late flagellar subunits after docking at the membrane ATPase of the type III export pathway

The bacterial flagellum assembles in a strict order, with structural subunits delivered to the growing flagellum by a type III export pathway. Early rod-and-hook subunits are exported before completion of the hook, at which point a subunit-specificity switch allows export of late filament subunits. This implies that in bacteria with multiple flagella at different stages of assembly, each export pathway can discriminate and sort unchaperoned early and chaperoned late subunits. To establish whether subunit sorting is distinct from subunit transition from the cytosol to the membrane, in particular docking at the membrane-associated FliI ATPase, the pathway was manipulated in vivo. When ATP hydrolysis by the FliI ATPase was disabled and when the pathway was locked into an early export state, both unchaperoned early and chaperoned late subunits stalled and accumulated at the inner membrane. Furthermore, a chaperone that attenuates late subunit export by stalling when docked at the wild-type ATPase also stalled at the ATPase in an early-locked pathway and inhibited export of early subunits in both native and early-locked pathways. These data indicate that the pathways for early and late subunits converge at the FliI ATPase, independent of ATP hydrolysis, before a distinct, separable sorting step. To ascertain the likely signals for sorting, the export of recombinant subunits was assayed. Late filament subunits unable to bind their chaperones were still sorted accurately, but chaperoned late subunits were directed through an early-locked pathway when fused to early subunit N-terminal export signal regions. Furthermore, while an early subunit signal directed export of a heterologous type III export substrate through both native and early-locked pathways, a late subunit signal only directed export via native pathways. These data suggest that subunits are distinguished not by late chaperones but by N-terminal export signals of the subunits themselves.

Analysis of functional domains present in the N-terminus of the SipB protein

SipB (593 aa), one of the Salmonella invasion proteins (Sips), is secreted via the Salmonella pathogenicity island 1 (SPI-1) type III secretion system (T3SS). Here, we report the delineation of several functional regions present in the SipB protein. Our data show that residues 3-8 of the SipB protein are essential for its secretion from the bacterial cell and that the SicA chaperone, which is important to ensure stability of SipB and SipC in the bacterial cytosol, binds to SipB somewhere between amino acids 80 and100 of the SipB N-terminal region. Interestingly, the N-terminal region (residues 1-160) of SipB (SipB160) cannot be secreted via the SPI-1 T3SS, but fusion of the C-terminal amphipathic region (residues 300-593) to SipB160 can restore secretion via this system.

EscC is a chaperone for the Edwardsiella tarda type III secretion system putative translocon components EseB and EseD

Edwardsiella tarda is a Gram-negative enteric pathogen that causes disease in both humans and animals. Recently, a type III secretion system (T3SS) has been found to contribute to Ed. tarda pathogenesis. EseB, EseC and EseD were shown to be secreted by the T3SS and to be the major components of the extracellular proteins (ECPs). Based on sequence similarity, they have been proposed to function as the 'translocon' of the T3SS needle structure. In this study, it was shown that EseB, EseC and EseD formed a protein complex after secretion, which is consistent with their possible roles as translocon components. The secretion of EseB and EseD was dependent on EscC (previously named Orf2). EscC has the characteristics of a chaperone; it is a small protein (13 kDa), located next to the translocators in the T3SS gene cluster, and has a coiled-coil structure at the N-terminal region as predicted by coils. An in-frame deletion of escC abolished the secretion of EseB and EseD, and complementation of DeltaescC restored the export of EseB and EseD into the culture supernatant. Further studies showed that EscC is not a secreted protein and is located on the membrane and in the cytoplasm. Mutation of escC did not affect the transcription of eseB but reduced the amount of EseB as measured by using an EseB-LacZ fusion protein in Ed. tarda. Co-purification studies demonstrated that EscC formed complexes with EseB and EseD. The results suggest that EscC functions as a T3SS chaperone for the putative translocon components EseB and EseD in Ed. tarda.

FliK regulates flagellar hook length as an internal ruler

The mechanism of length control of the flagellar hook is under debate between two theories. One claims that the FliK directly measures the hook length as a molecular ruler, while the other claims that the cytoplasmic substructure measures the amount of hook subunits to determine the hook length. Both agree that the FliK C-terminal domain catalyses the substrate-specificity switch to terminate hook elongation. In this study, we systematically created fliK mutants with deletions and insertions at various sites within the FliK N-terminal domain and analysed their effects on the final hook length. Insertions of peptide fragments from the Yersinia YscP into FliK gave rise to hooks with defined lengths, which was proportional to the molecular size of the FliK-YscP chimeras. Among fliK deletion mutants, only those with small truncations in three specific sites of FliK produced hooks of a defined, shortened length. For the majority of deletion mutants, FliK was secreted, but hook length was not controlled. On the other hand, for some deletion mutants FliK was not secreted, but the hook length was controlled, indicating that FliK secretion is not necessary for hook-length control. We conclude that FliK regulates hook length as an internal molecular ruler.

Bringing order to a complex molecular machine: the assembly of the bacterial flagella

The bacterial flagellum is an example of elegance in molecular engineering. Flagella dependent motility is a widespread and evolutionarily ancient trait. Diverse bacterial species have evolved unique structural adaptations enabling them to migrate in their environmental niche. Variability exists in the number, location and configuration of flagella, and reflects unique adaptations of the microorganism. The most detailed analysis of flagellar morphogenesis and structure has focused on Escherichia coli and Salmonella enterica. The appendage assembles sequentially from the inner to the outer-most structures. Additionally the temporal order of gene expression correlates with the assembly order of encoded proteins into the final structure. The bacterial flagellar apparatus includes an essential basal body complex that comprises the export machinery required for assembly of the hook and flagellar filament. A review outlining the current understanding of the protein interactions that make up this remarkable structure will be presented, and the associated temporal genetic regulation will be briefly discussed.

Identification of the flagellar chaperone FlgN in the phytopathogen Xanthomonas axonopodis pathovar citri by its interaction with hook-associated FlgK

Genome annotation of the plant pathogen Xanthomonas axonopodis pv. citri (Xac), identified flagellar genes in a 15.7 kb gene cluster. However, FlgN, a secretion chaperone for hook-associated proteins FlgK and FlgL, was not identified. We performed extensive screening of the X. axonopodis pv. citri genome with the yeast two-hybrid system to identify a protein with the characteristics of the flagellar chaperone FlgN. We found a candidate (XAC1990) encoded by an operon for components of the flagellum apparatus that interacted with FlgK. In order to further support this finding, Xac FlgK and XAC1990 were cloned, expressed, and purified. The recombinant proteins were characterized by spectroscopic methods and their interaction in vitro confirmed by pull-down assays. We, therefore, conclude that XAC1990 and its homologs in other Xanthomonas species are, in fact, FlgN proteins. These observations extend the sequence diversity covered by this family of proteins.

The type III secretion injectisome

The type III secretion injectisome is a complex nanomachine that allows bacteria to deliver protein effectors across eukaryotic cellular membranes. In recent years, significant progress has been made in our understanding of its structure, assembly and mode of operation. The principal structural components of the injectisome, from the base located in the bacterial cytosol to the tip of the needle protruding from the cell surface, have been investigated in detail. The structures of several constituent proteins were solved at the atomic level and important insights into the assembly process have been gained. However, despite the ongoing concerted efforts of molecular and structural biologists, the role of many of the constituent components of this nanomachine remain unknown.

An escort mechanism for cycling of export chaperones during flagellum assembly

Assembly of the bacterial flagellar filament requires a type III export pathway for ordered delivery of structural subunits from the cytosol to the cell surface. This is facilitated by transient interaction with chaperones that protect subunits and pilot them to dock at the membrane export ATPase complex. We reveal that the essential export protein FliJ has a novel chaperone escort function in the pathway, specifically recruiting unladen chaperones for the minor filament-class subunits of the filament cap and hook-filament junction substructures. FliJ did not recognize unchaperoned subunits or chaperone-subunit complexes, and it associated with the membrane ATPase complex, suggesting a function postdocking. Empty chaperones that were recruited by FliJ in vitro were efficiently captured from FliJ-chaperone complexes by cognate subunits. FliJ and subunit bound to the same region on the target chaperone, but the cognate subunit had a approximately 700-fold greater affinity for chaperone than did FliJ. The data show that FliJ recruits chaperones and transfers them to subunits, and indicate that this is driven by competition for a common binding site. This escort mechanism provides a means by which free export chaperones can be cycled after subunit release, establishing a new facet of the secretion process. As FliJ does not escort the chaperone for the major filament subunit, cycling may offer a mechanism for export selectivity and thus promote assembly of the junction and cap substructures required for initiation of flagellin polymerization.

FliT acts as an anti-FlhD2C2 factor in the transcriptional control of the flagellar regulon in Salmonella enterica serovar typhimurium

Flagellar operons are divided into three classes with respect to their transcriptional hierarchy in Salmonella enterica serovar Typhimurium. The class 1 gene products FlhD and FlhC act together in an FlhD(2)C(2) heterotetramer, which binds upstream of the class 2 promoters to facilitate binding of RNA polymerase. Class 2 expression is known to be enhanced by a disruption mutation in a flagellar gene, fliT. In this study, we purified FliT protein in a His-tagged form and showed that the protein prevented binding of FlhD(2)C(2) to the class 2 promoter and inhibited FlhD(2)C(2)-dependent transcription. Pull-down and far-Western blotting analyses revealed that the FliT protein was capable of binding to FlhD(2)C(2) and FlhC and not to FlhD alone. We conclude that FliT acts as an anti-FlhD(2)C(2) factor, which binds to FlhD(2)C(2) through interaction with the FlhC subunit and inhibits its binding to the class 2 promoter.

The flagellar-specific transcription factor, sigma28, is the Type III secretion chaperone for the flagellar-specific anti-sigma28 factor FlgM

The sigma(28) protein is a member of the bacterial sigma(70)-family of transcription factors that directs RNA polymerase to flagellar late (class 3) promoters. The sigma(28) protein is regulated in response to flagellar assembly by the anti-sigma(28) factor FlgM. FlgM inhibits sigma(28)-dependent transcription of genes whose products are needed late in assembly until the flagellar basal motor structure, the hook-basal body (HBB), is constructed. A second function for the sigma(28) transcription factor has been discovered: sigma(28) facilitates the secretion of FlgM through the HBB, acting as the FlgM Type III secretion chaperone. Transcription-specific mutants in sigma(28) were isolated that remained competent for FlgM-facilitated secretion separating the transcription and secretion-facilitation activities of sigma (28). Conversely, we also describe the isolation of mutants in sigma(28) that are specific for FlgM-facilitated secretion. The data demonstrate that sigma(28) is the Type III secretion chaperone for its own anti-sigma factor FlgM. Thus, a novel role for a sigma(70)-family transcription factor is described.

Novel conserved assembly factor of the bacterial flagellum

TP0658 (FliW) and its orthologs, conserved proteins of unknown function in Treponema pallidum and other species, interact with a C-terminal region of flagellin (FlaB1-3 in T. pallidum; FliC in most other species). Mutants of orthologs in Bacillus subtilis and Campylobacter jejuni (yviF, CJ1075) showed strongly reduced motility. TP0658 stabilizes flagellin in a way similar to FliS, suggesting that TP0658 is a conserved assembly factor for the bacterial flagellum.

Oligomerization of the bacterial flagellar ATPase FliI is controlled by its extreme N-terminal region

Salmonella FliI is the flagellar ATPase which converts the energy of ATP hydrolysis into the export of flagellar proteins. It forms a ring-shaped oligomer in the presence of ATP, its analogs, or phospholipids. The extreme N-terminal region of FliI has an unstable conformation and is responsible for the interaction with other components of the export apparatus and for regulation of the catalytic mechanism. To understand the role of this N-terminal region in more detail, we used multi-angle light-scattering, analytical ultracentrifugation, far-UV CD and biochemical methods to characterize a partially functional variant of FliI, missing its first seven amino acid residues (His-FliI(Delta1-7)), whose ATPase activity is about ten times lower than that of wild-type FliI. His-FliI(Delta1-7) is monomeric in solution. The deletion increased the content of alpha-helix, suggesting that the deletion stabilizes the unstable N-terminal region into an alpha-helical conformation. The deletion did not influence the K(m) value for ATP. However, unlike the wild-type, ATP and acidic phospholipids did not induce oligomerization of His-FliI(Delta1-7) or increase its ATPase activity. These results suggest that the deletion suppresses the oligomerization of FliI, and that a conformational change in the unstable N-terminal region is required for FliI oligomerization to effectively couple the energy of ATP hydrolysis to the translocation of flagellar proteins.

Identification of new flagellar genes of Salmonella enterica serovar Typhimurium

RNA levels of flagellar genes in eight different genetic backgrounds were compared to that of the wild type by DNA microarray analysis. Cluster analysis identified new, potential flagellar genes, three putative methyl-accepting chemotaxis proteins, STM3138 (McpA), STM3152 (McpB), and STM3216(McpC), and a CheV homolog, STM2314, in Salmonella, that are not found in Escherichia coli. Isolation and characterization of Mud-lac insertions in cheV, mcpB, mcpC, and the previously uncharacterized aer locus of S. enterica serovar Typhimurium revealed them to be controlled by sigma28-dependent flagellar class 3 promoters. In addition, the srfABC operon previously isolated as an SsrB-regulated operon clustered with the flagellar class 2 operon and was determined to be under FlhDC control. The previously unclassified fliB gene, encoding flagellin methylase, clustered as a class 2 gene, which was verified using reporter fusions, and the fliB transcriptional start site was identified by primer extension analysis. RNA levels of all flagellar genes were elevated in flgM or fliT null strains. RNA levels of class 3 flagellar genes were elevated in a fliS null strain, while deletion of the fliY, fliZ, or flk gene did not affect flagellar RNA levels relative to those of the wild type. The cafA (RNase G) and yhjH genes clustered with flagellar class 3 transcribed genes. Null alleles in cheV, mcpA, mcpB, mcpC, and srfB did not affect motility, while deletion of yhjH did result in reduced motility compared to that of the wild type.

The target cell plasma membrane is a critical interface for Salmonella cell entry effector-host interplay

Salmonella species trigger host membrane ruffling to force their internalization into non-phagocytic intestinal epithelial cells. This requires bacterial effector protein delivery into the target cell via a type III secretion system. Six translocated effectors manipulate cellular actin dynamics, but how their direct and indirect activities are spatially and temporally co-ordinated to promote productive cytoskeletal rearrangements remains essentially unexplored. To gain further insight into this process, we applied mechanical cell fractionation and immunofluorescence microscopy to systematically investigate the subcellular localization of epitope-tagged effectors in transiently transfected and Salmonella-infected cultured cells. Although five effectors contain no apparent membrane-targeting domains, all six localized exclusively in the target cell plasma membrane fraction and correspondingly were visualized at the cell periphery, from where they induced distinct effects on the actin cytoskeleton. Unexpectedly, no translocated effector pool was detectable in the cell cytosol. Using parallel in vitro assays, we demonstrate that the prenylated cellular GTPase Cdc42 is necessary and sufficient for membrane association of the Salmonella GTP exchange factor and GTPase-activating protein mimics SopE and SptP, which have no intrinsic lipid affinity. The data show that the host plasma membrane is a critical interface for effector-target interaction, and establish versatile systems to further dissect effector interplay.

Bacterial flagellar diversity in the post-genomic era

Flagellar biosynthesis has been studied most thoroughly in laboratory strains of Escherichia coli and Salmonella enterica. However, genome sequencing has uncovered flagellar loci in distantly related bacteria. We have used homology searches to determine how far the E. coli/S. enterica paradigm can be generalised to other flagellar systems. Numerous previously unrecognized homologues of flagellar components were discovered, including novel FlgM, FlgN, FliK and FliO homologues. Homology was found between the FliK proteins and a molecular ruler, YscP, from a virulence-associated type-III secretion system. Also described is a new family of flagellar proteins, the FlhX proteins, which resemble the cytoplasmic domain of FlhB.

Regulation of FlbD activity by flagellum assembly is accomplished through direct interaction with the trans-acting factor, FliX

The temporal and spatial transcription of late flagellar genes in Caulobacter crescentus is regulated by the sigma54 transcriptional activator, FlbD. One requirement for FlbD activity is the assembly of a structure encoded by early, class II flagellar genes. In this report, we show that the trans-acting factor FliX predominantly functions as a negative regulator of FlbD activity in the absence of the class II-encoded flagellar structure. In contrast, a mutant FliX that bypasses the transcriptional requirement for early flagellar assembly is incapable of repressing FlbD in a class II flagellar mutant. Expression of this mutant allele, fliX1, does not alter the temporal pattern of FlbD-dependent transcription. Remarkably, this mutation confers the correct cell cycle timing of hook operon transcription in a strain that cannot assemble the flagellum, indicating that the progression of flagellar assembly is a minor influence on temporal gene expression. Using a two-hybrid assay, we present evidence that FliX regulates FlbD through a direct interaction, a novel mechanism for this class of sigma54 transcriptional activator. Furthermore, increasing the cellular levels of FliX results in an increase in the concentration of FlbD, and a corresponding increase in FlbD-activated transcription, suggesting that FliX and FlbD form a stable complex in Caulobacter. FliX and FlbD homologues are present in several polar-flagellated bacteria, indicating that these proteins constitute an evolutionarily conserved regulatory pair in organisms where flagellar biogenesis is likely to be under control of the cell division cycle.

Crystal structure of the flagellar rotor protein FliN from Thermotoga maritima

FliN is a component of the bacterial flagellum that is present at levels of more than 100 copies and forms the bulk of the C ring, a drum-shaped structure at the inner end of the basal body. FliN interacts with FliG and FliM to form the rotor-mounted switch complex that controls clockwise-counterclockwise switching of the motor. In addition to its functions in motor rotation and switching, FliN is thought to have a role in the export of proteins that form the exterior structures of the flagellum (the rod, hook, and filament). Here, we describe the crystal structure of most of the FliN protein of Thermotoga maritima. FliN is a tightly intertwined dimer composed mostly of beta sheet. Several well-conserved hydrophobic residues form a nonpolar patch on the surface of the molecule. A mutation in the hydrophobic patch affected both flagellar assembly and switching, showing that this surface feature is important for FliN function. The association state of FliN in solution was studied by analytical ultracentrifugation, which provided clues to the higher-level organization of the protein. T. maritima FliN is primarily a dimer in solution, and T. maritima FliN and FliM together form a stable FliM(1)-FliN(4) complex. Escherichia coli FliN forms a stable tetramer in solution. The arrangement of FliN subunits in the tetramer was modeled by reference to the crystal structure of tetrameric HrcQB(C), a related protein that functions in virulence factor secretion in Pseudomonas syringae. The modeled tetramer is elongated, with approximate dimensions of 110 by 40 by 35 Angstroms, and it has a large hydrophobic cleft formed from the hydrophobic patches on the dimers. On the basis of the present data and available electron microscopic images, we propose a model for the organization of FliN subunits in the C ring.

Analysis of Pseudomonas fluorescens F113 genes implicated in flagellar filament synthesis and their role in competitive root colonization

The ability of plant-associated micro-organisms to colonize and compete in the rhizosphere is specially relevant for the biotechnological application of micro-organisms as inoculants. Pseudomonads are one of the best root colonizers and they are widely used in plant-pathogen biocontrol and in soil bioremediation. This study analyses the motility mechanism of the well-known biocontrol strain Pseudomonas fluorescens F113. A 6.5 kb region involved in the flagellar filament synthesis, containing the fliC, flaG, fliD, fliS, fliT and fleQ genes and part of the fleS gene, was sequenced and mutants in this region were made. Several non-motile mutants affected in the fliC, fliS and fleQ genes, and a fliT mutant with reduced motility properties, were obtained. These mutants were completely displaced from the root tip when competing with the wild-type F113 strain, indicating that the wild-type motility properties are necessary for competitive root colonization. A mutant affected in the flaG gene had longer flagella, but the same motility and colonization properties as the wild-type. However, in rich medium or in the absence of iron limitation, it showed a higher motility, suggesting the possibility of improving competitive root colonization by manipulating the motility processes.

Type III protein secretion mechanism in mammalian and plant pathogens

The type III protein secretion system (TTSS) is a complex organelle in the envelope of many Gram-negative bacteria; it delivers potentially hundreds of structurally diverse bacterial virulence proteins into plant and animal cells to modulate host cellular functions. Recent studies have revealed several basic features of this secretion system, including assembly of needle/pilus-like secretion structures, formation of putative translocation pores in the host membrane, recognition of N-terminal/5' mRNA-based secretion signals, and requirement of small chaperone proteins for optimal delivery and/or expression of effector proteins. Although most of our knowledge about the TTSS is derived from studies of mammalian pathogenic bacteria, similar and unique features are learned from studies of plant pathogenic bacteria. Here, we summarize the most salient aspects of the TTSS, with special emphasis on recent findings.

Type III flagellar protein export and flagellar assembly

Bacterial flagella, unlike eukaryotic flagella, are largely external to the cell and therefore many of their subunits have to be exported. Export is ATP-driven. In Salmonella, the bacterium on which this chapter largely focuses, the apparatus responsible for flagellar protein export consists of six membrane components, three soluble components and several substrate-specific chaperones. Other flagellated eubacteria have similar systems. The membrane components of the export apparatus are housed within the flagellar basal body and deliver their substrates into a channel or lumen in the nascent structure from which point they diffuse to the far end and assemble. Both on the basis of sequence similarities of several components and structural similarities, the flagellar protein export systems clearly belong to the type III superfamily, whose other members are responsible for secretion of virulence factors by many species of pathogenic bacteria.

Self-assembly and type III protein export of the bacterial flagellum

The bacterial flagellum is a supramolecular structure consisting of a basal body, a hook and a filament. Most of the flagellar components are translocated across the cytoplasmic membrane by the flagellar type III protein export apparatus in the vicinity of the flagellar base, diffuse down the narrow channel through the nascent structure and self-assemble at its distal end with the help of a cap structure. Flagellar proteins synthesized in the cytoplasm are targeted to the export apparatus with the help of flagellum-specific chaperones and pushed into the channel by an ATPase, whose activity is controlled by its regulator to enable the energy of ATP hydrolysis to be efficiently coupled to the translocation reaction. The export apparatus switches its substrate specificity by monitoring the state of flagellar assembly in the cell exterior, allowing this huge and complex macromolecular assembly to be built efficiently by a highly ordered and well-regulated assembly process.